The present invention relates to a system and a method for improving conversion efficiency of a lean NOx catalyst in a diesel or lean burn gasoline engine, and, more particularly, to improving conversion efficiency by controlling delivery of a NOx reductant.
Internal combustion engines commonly rely on exhaust aftertreatment devices to convert regulated components: carbon monoxide, hydrocarbons, and nitrogen oxides (NOx), into carbon dioxide, water, nitrogen, and oxygen. Exhaust catalysts have been extensively developed to obtain high conversion efficiencies on stoichiometric exhaust gases. Stoichiometric conditions are achieved when the fuel and oxidizer supplied to the engine is in a proportion which, if reaction of the fuel were complete, produce carbon dioxide, water, and nitrogen. It is known to those skilled in the art, though, that higher fuel efficiency is obtained from engines operating at air-fuel ratios lean of stoichiometric, that is, with an excess of air. These lean burning engines may be diesel engines, stratified-charge gasoline engines in which the fuel and air are only partially mixed, and homogeneous-charge, lean-burn gasoline engines in which the fuel and air are mostly premixed prior to combustion. Because of the desire for high fuel efficiency, lean burning engines are in production and continue to be developed. It is known to those skilled in the art to use a NOx catalyst and continuously supply reductant to the catalyst to convert NOx while operating lean.
The problems with prior art methods are that some of the reductant supplied to the catalyst slips through the catalyst unreacted and NOx conversion is too low at temperatures below about 250° C.
The inventors herein have recognized a method which uses a lesser amount of reductant, limits slippage of unreacted reductant through the catalyst, and significantly increases NOx conversion efficiency of the lean NOx catalyst in the 140-250° C. temperature range.